Note: Descriptions are shown in the official language in which they were submitted.
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APPARATUS AND METHOD FOR FORMATION EVALUATION
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to techniques for performing formation
evaluation of a
subterranean formation by a down hole tool positioned in a well bore
penetrating the
subterranean formation. More particularly, but not by way of limitation, the
present
invention relates to techniques for determining fluid parameters, such as the
viscosity and
density of formation fluid drawn into and/or evaluated by the down hole tool.
2. Background of the Related Art
Well bores are drilled to locate and produce hydrocarbons. A down hole
drilling tool
with a bit at an end thereof is advanced into the ground to form a well bore.
As the drilling
tool is advanced, drilling mud is pumped through the drilling tool and out the
drill bit to cool
the drilling tool and carry away cuttings. The drilling mud additionally forms
a mud cake
that lines the well bore.
During the drilling operation, it is desirable to perform various evaluations
of the
formations penetrated by the well bore. In some cases, the drilling tool may
be removed and
a wire line tool may be deployed into the well bore to test and/or sample the
formation. In
other cases, the drilling tool may be provided with devices to test and/or
sample the
surrounding formation and the drilling tool may be used to perform the testing
or sampling.
These samples or tests may be used, for example, to locate valuable
hydrocarbons.
Formation evaluation often requires that fluid from the formation be drawn
into the
down hole tool for testing and/or sampling. Various devices, such as probes,
are extended
from the down hole tool to establish fluid communication with the formation
surrounding the
well bore and to draw fluid into the down hole tool. A typical probe is a
circular element
extended from the down hole tool and positioned against the sidewall of the
well bore. A
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rubber packer at the end of the probe is used to create a seal with the wall
of the well bore.
Another device used to form a seal with the well bore is referred to as a dual
packer. With a
dual packer, two elastomeric rings expand radially about the tool to isolate a
portion of the
well bore there between. The rings form a seal with the well bore wall and
permit fluid to be
drawn into the isolated portion of the well bore and into an inlet in the down
hole tool.
The mud cake lining the well bore is often useful in assisting the probe
and/or dual
packers in making the seal with the well bore wall. Once the seal is made,
fluid from the
formation is drawn into the down hole tool through an inlet by lowering the
pressure in the
down hole tool. Examples of probes and/or packers used in down hole tools are
described in
U.S. Patent Nos. 6,301,959; 4,860,581; 4,936,139; 6,585,045; 6,609,568 and
6,719,049 and
U.S. Patent Application No. 2004/0000433.
Formation evaluation is typically performed on fluids drawn into the down hole
tool.
Techniques currently exist for performing various measurements, pretests
and/or sample
collection of fluids that enter the down hole tool. However, it has been
discovered that when
the formation fluid passes into the down hole tool, various contaminants, such
as well bore
fluids and/or drilling mud, may enter the tool with the formation fluids.
These contaminates
may affect the quality of measurements and/or samples of the formation fluids.
Moreover,
contamination may cause costly delays in the well bore operations by requiring
additional
time for more testing and/or sampling. Additionally, such problems may yield
false results
that are erroneous and/or unusable.
It is, therefore, desirable that the formation fluid entering into the down
hole tool be
sufficiently "clean" or "virgin" for valid testing. In other words, the
formation fluid should
have little or no contamination. Attempts have been made to eliminate
contaminates from
entering the down hole tool with the formation fluid. For example, as depicted
in US Patent
No. 4,951,749, filters have been positioned in probes to block contaminates
from entering the
down hole tool with the formation fluid. Additionally, as shown in US Patent
No. 6,301,959
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issued to Hrametz, a probe is provided with a guard ring to divert
contaminated fluids away
from clean fluid as it enters the probe. Fluid entering the down hole tool
typically passes
through flow lines and may be captured in a sample chamber or dumped into the
well bore.
Various valves, gauges and other components may be incorporated along the flow
lines to
divert, test and/or capture the fluid as it passes through the down hole tool.
Fluid passing through the down hole tool may be tested to determine various
down
hole parameters or properties. The thermophysical properties of hydrocarbon
reservoir
fluids, such as viscosity, density and phase behavior of the fluid at
reservoir conditions, may
be used to evaluate potential reserves, determine flow in porous media and
design
completion, separation, treating, and metering systems, among others.
Various techniques have been developed for determining viscosity of fluids.
For
example, viscometers having a bob suspended between fixation points for a
torsion wire have
also been proposed as described, for example, in U.S. Patent Nos. 5,763,766
and 6,070,457.
Viscometers have also been formed from vibrating objects. One such viscometer
has been
used in down hole applications for measuring the viscosity, density and
dielectric constant of
formation fluid or filtrate in a hydrocarbon producing well. For example,
International
Publication Number WO 02/093126 discloses a tuning fork resonator within a
pipe to provide
real-time direct measurements and estimates of the viscosity, density and
dielectric constant
of formation fluid or filtrate within the hydrocarbon producing well. Another
viscometer,
having a wire clamped between two posts has been used in a laboratory
environment as
described, for example in The Viscosity of Pressurized He above T~, Physica 76
(1974) 177-
180; Vibrating Wire Viscometer, The Review of Scientific Instruments Vol. 35,
No. 10
(October 1964) pgs. 1345-1348.
Despite the existence of techniques for measuring viscosity, there remains a
need to
provide accurate viscosity measurements down hole, and preferably without
regard to the
position of a sensor down hole relative to the gravitational field. It is
desirable that such a
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system be capable of providing checks for precision and/or accuracy. It is
further desirable
that such a system be provided with a simple configuration adapted for use in
a harsh well
bore environment.
SUMMARY OF THE INVENTION
In one aspect, the present invention relates to a viscometer-densimeter for a
down
hole tool positionable in a well bore penetrating a subterranean formation.
The down hole
tool is adapted to convey at least a portion of a fluid in the formation to
the viscometer-
densimeter. The viscometer-densimeter includes a sensor unit positionable
within the down
hole tool. The sensor unit includes at least two spatially disposed
connectors, a wire, and at
least one magnet. The wire is suspended in tension between the at least two
connectors such
that the wire is available for interaction with the fluid when the viscometer-
densimeter is
positioned within the down hole tool and the down hole tool is positioned
within the
subterranean formation and receives the fluid from the subterranean formation.
The
connectors and the wire are constructed so as to provide a frequency
oscillator. The at least
one magnet emits a magnetic field interacting with the wire.
The connectors and the wire can be constructed of materials having similar
coefficients of thermal expansion so as to provide the frequency oscillator.
For example, the
connectors and the wire can be constructed of a single type of material to
substantially
eliminate variations in the resonant frequency of the wire due to thermal and
elastic
deformation caused by down hole conditions. The viscometer-densimeter can also
be
provided with a flow tube in which the wire is suspended by the connectors,
and in this
instance the flow tube, the connectors and the wire are desirably constructed
of materials
having similar coefficients of thermal expansion so as to provide the
frequency oscillator.
In another aspect, the sensor unit is further provided with a means for
preventing
rotation of the wire with respect to the connectors. The means for preventing
rotation of the
wire can include a boss connected to the wire with the boss having a non-
circular cross-
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section.
In yet another aspect, the viscometer-densimeter is further provided with an
analytical
circuit receiving feedback from the wire for calculating at least two
parameters (e.g.,
viscosity and density) of fluid interacting with the wire.
In yet another aspect, the present invention relates to a down hole tool
positionable in
a well bore having a wall and penetrating a subterranean formation. The
formation typically
has a fluid, such as natural gas or oil therein. The down hole tool is
provided with a housing,
a fluid communication device, and a viscometer-densimeter. The housing
encloses at least
one evaluation cavity. The fluid communication device is extendable from the
housing for
sealing engagement with the wall of the well bore. The fluid communication
device has at
least one inlet communicating with the evaluation cavity for receiving the
fluid from the
formation and depositing such fluid into the evaluation cavity. The viscometer-
densimeter is
provided with a sensor unit positioned within the evaluation cavity. The
sensor unit is
provided with at least two spatially disposed connectors, a wire and a magnet.
The wire is
suspended in tension between the at least two connectors such that the wire is
available for
interaction with the fluid within the evaluation cavity. The connectors and
the wire are
constructed so as to provide a frequency oscillator. The at least one magnet
emits a magnetic
field interacting with the wire. The viscometer can be any of the versions
discussed above.
In yet another aspect, the down hole tool can be provided with a comparison
chamber
containing a fluid of known properties, e.g., viscosity and density. The down
hole conditions,
e.g., pressure and temperature, within the comparison chamber are similar (and
preferably
identical) to the down hole conditions within the evaluation cavity. The down
hole tool is also
provided with a sensor unit within the comparison chamber such that the down
hole includes
one sensor unit positioned within a fluid of unknown parameters within the
evaluation cavity
and the other sensor unit positioned with a fluid of known parameters within
the comparison
chamber. A signal indicative of at least two of the unknown parameters of the
fluid (e.g.,
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viscosity and density) within the evaluation cavity is then computed.
In a fiu-ther aspect, the present invention relates to a method for measuring
viscosity
and density parameters of an unknown fluid within a well bore penetrating a
forniation having
the fluid therein. In this method, a fluid communication device of the down
hole tool is
positioned in sealing engagement with a wall of the well bore. Fluid is then
drawn out of the
formation and into an evaluation cavity within the down hole tool. Data of the
fluid within
the evaluation cavity is sampled with a viscometer-densimeter having a wire
positioned
within the evaluation cavity and suspended between two connectors. The wire
and the
connectors are constructed to provide a frequency oscillator.
In this aspect, the evaluation cavity can be a flow-line or a sample chamber.
With the
data sampled by the viscometer-densimeter, at least two parameters can be
calculated
utilizing the data sampled within the evaluation cavity. The at least two
parameters include
viscosity and density.
In yet another aspect, the method can further comprise the step of sampling
data with
respect to a known fluid within a comparison chamber having a temperature and
pressure
related to the temperature and pressure of the fluid within the evaluation
cavity. In this
instance, the method typically further includes the step of calculating at
least two parameters
of the unknown fluid within the evaluation cavity utilizing the data sampled
from the
comparison chamber and the data sampled from the evaluation cavity.
In a further aspect, the present invention relates to a computer readable
medium which
can be either provided to or included in an analytical circuit for calculating
at least two fluid
parameters, such as the viscosity and density of the fluid. In this instance,
the computer
readable medium includes logic for (1) receiving feedback from at least two
sensor units with
one sensor unit positioned within a fluid of unknown parameters and the other
sensor unit
positioned with a fluid of known parameters, and (2) computing a signal
indicative of at least
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two of the unknown parameters of the fluid in which the one sensor unit is
positioned while
substantially eliminating variations in the well bore conditions surrounding
the sensor unit
within the fluid of unknown parameters. The logic for computing the signal can
include, for
example, logic for performing a joint inversion of the data received from the
sensor units.
In each of the aspects of the present invention recited above, the at least
two fluid
parameters are preferably calculated simultaneously.
BRIEF DESCRIPTION OF THE DRAWINGS
So that the above recited features and advantages of the present invention can
be
understood in detail, a more particular description of the invention, briefly
summarized
above, may be had by reference to the embodiments thereof that are illustrated
in the
appended drawings. It is to be noted, however, that the appended drawings
illustrate only
typical embodiments of this invention and are therefore not to be considered
limiting of its
scope, for the invention may admit to other equally effective embodiments.
Figure 1 is a schematic, partial cross-sectional view of a down hole wire line
tool
having an internal viscometer-densimeter with the wire line tool suspended
from a rig.
Figure 2 is a schematic, partial cross-sectional view of a down hole drilling
tool
having an internal viscometer-densimeter with the down hole drilling tool
suspended from a
rig.
Figure 3A is a schematic representation of a portion of the down hole tool of
Figure 1
having a probe registered against a sidewall of the well bore and a viscometer-
densimeter
positioned within an evaluation flow line within the down hole tool.
Figure 3B is a schematic representation of another version of the down hole
tool of
Figure 1 having a cleanup flow line utilized in combination with a dual
packer.
Figure 4 is a side elevation of a viscometer-densimeter positioned within an
evaluation cavity.
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Figure 5 is a cross-sectional view of a sensor unit of the viscometer-
densimeter of
Figure 4 showing a suspended wire.
Figure 6 is an exploded perspective view of the sensor unit of the viscometer-
densimeter depicted in Figure 4.
Figure 7a is a logic flow diagram illustrating a method for simultaneously
calculating
viscosity and density.
Figure 7b is a logic flow diagram illustrating another method for
simultaneously
calculating viscosity and density.
Figure 8 is a graph illustrating a chi-square performance surface intercepted
by a
fixed fo hyper plane showing a minimum utilized in the calculation of the
density and
viscosity.
Figure 9 is an exploded perspective view of another sensor unit of a
viscometer-
densimeter.
Figure 10 is a top plan view of the sensor unit depicted in Figure 9.
Figure 11 is a side-elevational view of another version of a sensor unit.
Figure 12 is a cross-sectional view of the sensor unit of Figure 11 taken
along the
lines 12--12 in Figure 11.
Figure 13 is a fragmental, schematic representation of another version of a
down hole
tool having two or more viscometer-densimeters with one of the viscometer-
densimeters
positioned within a fluid of unknown viscosity and density and another one of
the
viscometer-densimeters positioned within a fluid of known viscosity and
density.
Figure 14a is a logic flow diagram illustrating a method for simultaneously
calculating viscosity and density utilizing the arrangement shown in Figure
13.
Figure 14b is a logic flow diagram illustrating another method for
simultaneously
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calculating viscosity and density utilizing the arrangement shown in Figure
13.
DETAILED DESCRIPTION OF THE INVENTION
Presently preferred embodiments of the invention are shown in the above-
identified
figures and described in detail below. In describing the preferred
embodiments, like or
identical reference numerals are used to identify common or similar elements.
The figures
are not necessarily to scale and certain features and certain views of the
figures may be
shown exaggerated in scale or in schematic in the interest of clarity and
conciseness.
DEFINITIONS
Certain terms are defined throughout this description as they are first used,
while
certain other terms used in this description are defined below:
"Annular" means of, relating to, or forming a ring, i.e., a line, band, or
arrangement in
the shape of a closed curve such as a circle or an ellipse.
"Contaminated fluid" means a fluid, e.g., gas or a liquid, that is generally
unacceptable for hydrocarbon fluid sampling and/or evaluation because the
fluid contains
contaminates, such as filtrate from the mud utilized in drilling the bore
hole.
"Down hole tool" means tools deployed into the well bore by means such as a
drill
string, wire line, and coiled tubing for performing down hole operations
related to the
evaluation, production, and/or management of one or more subsurface formations
of interest.
"Operatively connected" means directly or indirectly connected for
transmitting or
conducting information, force, energy, or matter (including fluids).
"Virgin fluid" means subsurface fluid, e.g., gas or a liquid, that is
sufficiently pure,
pristine, connate, uncontaminated or otherwise considered in the fluid
sampling and analysis
field to be acceptably representative of a given formation for valid
hydrocarbon sampling
and/or evaluation.
"Fluid" means either "virgin fluid" or "contaminated fluid."
"Clamp" means a device designed to bind or constrict or to press two or more
parts
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together so as to hold them firmly.
"Connector" means any device or assembly, such as a clamp, for rigidly joining
or
gripping a portion of a wire.
"frequency oscillator" means the resonant frequency of a tensioned wire in
vacuum
(hereinafter referred to as ' f,") is predictable so that changes in well bore
conditions, e.g.,
temperature and pressure, do not have a substantial effect on the resonant
frequency of the
tensioned wire whereby readings obtained from the tensioned wire in varying
well bore
conditions are acceptably representative of the characteristics of the fluid
interacting with the
tensioned wire.
DETAILED DESCRIPTION
Figure 1 depicts a down hole tool 10 constructed in accordance with the
present
invention suspended from a rig 12 into a well bore 14. The down hole tool 10
can be any
type of tool capable of performing formation evaluation, such as drilling,
coiled tubing or
other down hole tool. The down hole tool 10 of Figure 1 is a conventional wire
line tool
deployed from the rig 12 into the well bore 14 via a wire line cable 16 and
positioned
adjacent to a formation F. The down hole tool 10 is provided with a probe 18
adapted to seal
with a wall 20 of the well bore 14 (hereinafter referred to as a"wal120" or
"well bore wall
20") and draw fluid from the formation F into the down hole tool 10 as
depicted by the
arrows. Backup pistons 22 and 24 assist in pushing the probe 18 of the down
hole tool 10
against the well bore wall 20.
Figure 2 depicts another example of a down hole tool 30 constructed in
accordance
with the present invention. The down hole too130 of Figure 2 is a drilling
tool, which can be
conveyed among one or more (or itself may be) a measurement-while-drilling
(MWD)
drilling tool, a logging-while-drilling (LWD) drilling tool, or other drilling
tool that is known
to those skilled in the art. The down hole tool 30 is attached to a drill
string 32 driven by the
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rig 12 to form the well bore 14. The down hole tool 30 includes the probe 18
adapted to seal
with the wall 20 of the well bore 14 to draw fluid from the formation F into
the down hole
too130 as depicted by the arrows. The viscometer-densimeters or sensor units
described
below can be used with either the down hole tool 10 or the down hole too130.
Figure 3A is a schematic view of a portion of the down hole tool 10 of Figure
1
depicting a fluid flow system 34. The probe 18 is preferably extended from a
housing 35 of
the down hole tool 10 for engagement with the well bore wall 20. The probe 18
is provided
with a packer 36 for sealing with the well bore wal120. The packer 36 contacts
the well bore
wall 20 and forms a seal with a mud cake 401ining the well bore 14. The mud
cake 40 seeps
into the well bore wall 20 and creates an invaded zone 42 about the well bore
14. The
invaded zone 42 contains mud and other well bore fluids that contaminate the
surrounding
formations, including the formation F and a portion of the virgin fluid 44
contained therein.
The probe 18 is preferably provided with an evaluation flow line 46. Examples
of
fluid communication devices, such as probes and dual packers, used for drawing
fluid into a
flow line are depicted in US Patent Nos. 4,860,581 and 4,936,139.
The evaluation flow line 46 extends into the down hole tool 10 and is used to
pass
fluid, such as virgin fluid 44 into the down hole tool 10 for testing and/or
sampling. The
evaluation flow line 46 extends to a sample chamber 50 for collecting samples
of the virgin
fluid 44. A pump 52 may be used to draw fluid through the flow line 46.
While Figure 3A shows a sample configuration of a down hole tool used to draw
fluid
from a formation, it will be appreciated by one of ordinary skill in the art
that a variety of
configurations of probes, flow lines and down hole tools may be used and is
not intended to
limit the scope of the invention.
For example, Figure 3B is a schematic view of a portion of another version of
the
down hole tool 10 having a modified probe 18a and a fluid flow system 34a for
drawing fluid
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into separate flow lines. More specifically, the fluid flow system 34a
depicted in Figure 3B is
similar to the fluid flow system 34 depicted in Figure 3A, except that the
fluid flow system
34a includes a cleanup flow line 46a in addition to the evaluation flow line
46 as well as
pumps 52a and 52b associated with the respective flow lines 46 and 46a. The
probe 18a
depicted in Figure 3B is similar to the probe 18 depicted in Figure 3A, except
that the probe
18a has two separate cavities 56a and 56b with the cavity 56a communicating
with the flow
line 46 and the cavity 56b communicating with the flow line 46a. The cavity
56b extends
about the cavity 56a such that the cavity 56b draws "contaminated fluid" from
the formation
F to permit the cavity 56a to draw "virgin fluid" from the formation F. The
contaminated
fluid is expelled from the cleanup flow line 46a into the well bore 14 through
an outlet 57.
Examples of fluid communication devices, such as probes and dual packers, used
for drawing
fluid into separate flow lines are depicted in US Patent No. 6,719,049 and US
Published
Application No. 20040000433, assigned to the assignee of the present
invention, and US
Patent No. 6,301,959 assigned to Halliburton.
In accordance with the present invention, a viscometer-densimeter 60 (a, b, c)
is
associated with an evaluation cavity within the down hole tool 10, such as the
evaluation flow
line 46, the cleanup flow line 46a, or the sample chamber 50 for measuring the
viscosity of
the fluid within the evaluation cavity. In the example depicted in Figure 3B,
the viscometer-
densimeter 60 is labeled with the reference numerals 60a, 60b and 60c for
purposes of clarity.
The viscometer-densimeter 60 is shown in more detail in Figures 4, 5 and 6.
The down hole tool 30 may also provided with the housing, the probe, the fluid
flow
system, the packer, the evaluation flow line, the cleanup flow line, the
sample chamber, the
pump(s) and the viscometer-densimeter(s) in a similar manner as the versions
of the down
hole tool 10 depicted in Figures 3A and 3B.
Referring now to Figures 4-6, the viscometer-densimeter 60 will be described
in detail
hereinafter with respect to the evaluation cavity being within the evaluation
flow line 46.
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However, it should be understood that the following description is equally
applicable to the
evaluation cavity being within the cleanup flow line 46a or the sample chamber
50. It should
also be understood that although the viscometer-densimeter 60 will be
described in
conjunction with the down hole tool 10, such description is equally applicable
to the down
hole tool 30. Moreover, while the viscometer-densimeter 60 is depicted in
Figures 3A and
3B positioned along flow lines 46 and 46a, the viscometer-densimeter 60 may be
positioned
in various locations about the down hole tool 10 for measuring down hole
parameters.
In general, the viscometer-densimeter 60 has a sensor unit 62, one or more
magnets
64(a, b), a signal processor 66, and an analytical circuit 68. In the example
shown in Figure
4, the viscometer-densimeter 60 is provided with two magnets which are
designated in Figure
4 by the reference numerals 64a and 64b. The sensor unit 62 is provided with
at least two
spatially disposed connectors 72, and a wire 74 (Figure 5) suspended between
the least two
connectors 72 such that the wire 74 is available for interaction with the
fluid when the sensor
unit 62 of the viscometer-densimeter 60 positioned within the down hole tool
10 and the
down hole tool 10 is positioned within the subterranean formation F and
receives the fluid
from the formation F. The magnets 64a and 64b emit a magnetic field, which
interacts with
the sinusoidal current flowing through the wire 74. The signal processor 66
electrically
communicates with the wire 74 via signal paths 75a and 75b. The signal paths
75a and 75b
can be wire, cable or air-way communication links. The signal processor 66
provides a drive
voltage forming a sinusoidal current to the wire 74, which typically causes
the wire 74 to
vibrate or resonate consistent with the signal provided thereto. Typically,
the signal provided
to the wire 74 from the signal processor 66 can be considered a swept
frequency constant
current signal wherein the frequency of the signal is changing in a
predetermined manner.
The analytical circuit 68 receives feedback from the wire 74. The sinusoidal
current
flows through the wire 74 and when the frequency is close to that of a
resonance, typically
the lowest order mode, a detectable motional electromotive force ("emf ") is
generated. It is
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the drive voltage and the motional emf that are measured as a function of
frequency over the
resonance. Typically, the analytical circuit 68 receives feedback from the
wire 74 indicative
of the resonant frequency of the wire 74. Depending upon the viscosity of the
fluid, the
resonant frequency of the wire 74 changes in a predictable manner, which
allows for the
determination of the viscosity of the fluid. The manner in which the viscosity
is determined
from the feedback from the wire 74 will be discussed in more detail below. The
analytical
circuit 68 can be any type of circuit capable of receiving feedback from the
wire 74 and
calculating the viscosity of the fluid. Typically, the analytical circuit 68
will include a
computer processor executing a software program stored on a computer readable
medium
such as a memory or a disk, for permitting the analytical circuit 68 to
calculate the viscosity.
However, it should be understood that in certain embodiments, the analytical
circuit 68 could
be implemented using analog, or other types of devices. For example, the
analytical circuit
68 may include an analog-to-digital converter followed by a decoder for
calculating the
viscosity of the fluid. Although the analytical circuit 68 and signal
processor 66 have been
shown in Figure 4 separately, it should be understood that the analytical
circuit 68 and the
signal processor 66 can be implemented in a single circuit, or implemented in
separate
circuits. Furthermore, although the analytical circuit 68 and the signal
processor 66 are
illustrated in Figure 4 as being within the down hole tool 10, it should be
understood that the
signal processor 66 and/or the analytical circuit 68 could be located external
to the down hole
tool 10. For example, the signal processor 66 for generating the swept signal
can be located
within the down hole tool 10, while the analytical circuit 68 is located
outside of the well
bore 14 in a monitoring center located either near the well bore 14 or remote
from the well
bore 14.
The sensor unit 62 of the viscometer-densimeter 60 is also provided with a
housing
76. The housing 76 defines a channe178 (Figures 5 and 6), an inlet 80
communicating with
the channel 78, and an outlet 82 communicating with the channel 78. In the
example
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depicted in Figure 4, the fluid is flowing in a direction 84 through the
evaluation flow line 46.
Thus, when the fluid encounters the sensor unit 62, the fluid flows through
the inlet 80, into
the channel 78 and exits the housing 76 through the outlet 82. When the
housing 76 is
provided with an outer dimension smaller than an inner dimension of the
evaluation flow line
46, a certain amount of the fluid will also flow past the housing 76 in a
channel 87 (Figure 4)
formed between an outer surface 88 of the housing 76, and an inner surface 89
of the
evaluation flow line 46.
The wire 74 is positioned within the channel 78 so that the fluid will come
into
contact with substantially the entire wire 74 between the connectors 72 as the
fluid passes
through the housing 76. This ensures that the fluid flows over the entire
length of the wire 74
between the connectors 72 to facilitate cleaning the wire 74 between fluids.
The wire 74 is
constructed of a conductive material capable of vibrating at a plurality of
fundamental mode
resonant frequencies (or harmonics thereof) depending upon the tension of the
wire 74 and
the viscosity of the fluid surrounding the wire 74. The wire 74 is desirably
constructed of a
material having a large density because the greater the difference in density
of the wire 74 to
that of the fluid the greater the sensitivity. The wire 74 also needs to have
a high Young's
modulus to provide a stable resonance while the density provides sensitivity
to the fluid
around it, through the ratio of the density of the fluid to the density of the
wire. A variety of
materials can be used for the wire 74. For example, the wire 74 can be
constructed of
Tungsten or Chromel. When the wire 74 is used for sensing a gas, such as
natural gas, it is
preferred that the wire 74 have a relatively smooth outer surface. In this
instance Chromel is
a preferred material for constructing the wire 74.
As shown in Figure 4, the magnets 64 are preferably positioned on the exterior
of the
evaluation flow line 46 and mounted to an exterior surface of the evaluation
flow line 46.
The magnets 64 can also be incorporated into the housing 76. Alternatively,
the housing 76
can be constructed of a magnetic material.
CA 02528817 2005-12-01
As shown in Figures 5 and 6, the housing 76 may be provided with a first
housing
member 90 and a second housing member 92. The first housing member 90 and the
second
housing member 92 cooperate to define the channel 78. The first housing member
90 and the
second housing member 92 are preferably constructed of a conductive, non-
magnetic material
such that the magnetic field generated by the magnets 64 can interact with the
wire 74
without substantial interference from the housing 76. For example, the first
housing member
90 and the second housing member 92 may be constructed of a down hole
compatible
material, such as K500 Monel, tungsten or another type of non-magnetic
material, e.g.,
stainless steel.
The housing 76 is also provided with an insulating layer 96 (Figure 5)
positioned
between the first housing member 90 and the second housing member 92 so as to
electrically
isolate the first housing member 90 from the second housing member 92. The
wire 74
extends between opposite sides of the insulating layer 96 to electrically
connect the first
housing member 90 to the second housing member 92. The insulating layer 96 may
be
constructed of a first insulating member 98, and a second insulating member
100. The wire
74 is provided with a first end 102, and a second end 104. The first
insulating member 98 is
positioned adjacent to the first end 102 of the wire 74, and the second
insulating member 100
is positioned adjacent to the second end 104 of the wire 74. The wire 74 spans
the channel 78
and serves to electrically connect the first housing member 90 to the second
housing member
92.
In the example of the sensor unit 62 depicted in Figure 4, each of the first
housing
member 90 and the second housing member 92 can be characterized as having a
first end
portion 108, a second end portion 110, and a medial portion 112 positioned
between the first
end portion 108 and the second end portion 110. The first end portion 108 and
the second
end portion 110 are provided with a cross-sectional area, or diameter which is
less than a
cross-sectional area or diameter of the medial portion 112. Thus, each of the
first housing
16
CA 02528817 2005-12-01
member 90, and the second housing member 92 has a shoulder 114 separating the
first end
portion 108 and the second end portion 110 from the medial portion 112. The
inlet 80 and
the outlet 82 are defined in the first housing member 90 and the second
housing member 92
proximate to the shoulders 114 such that the channel 78 extends through the
medial portion
112 of the housing 76. The shoulders 114 are shaped to direct the fluid into
the inlet 80.
To connect the signal paths 75a and 75b to the sensor unit 62, the viscometer-
densimeter 60 is further provided with a first terminal 116 coupled to the
first housing
member 90 and a second terminal 118 coupled to the second housing member 92.
The signal
processor 66 and the analytical circuit 68 are thus in communication with the
first and second
terminals 116 and 118 via the signal paths 75a and 75b. It should be noted
that the signal
paths 75a and 75b typically extend through the evaluation flow line 46 via one
or more feed-
throughs 120. The feed-throughs 120 provide a fluid tight seal to permit the
signal paths 75a
and 75b to extend through the evaluation flow line 46 while preventing fluid
from flowing
through the opening formed in the evaluation flow line 46.
The first terminal 116 and the second terminal 118 may be identical in
construction
and function. To implement the first terminal 116 and the second terminal 118,
the first
housing member 90 and the second housing member 92 can be provided with
threaded holes
124 formed in either the first end portion 108 or the second end portion 110
of the first
housing member 90 and the second housing member 92. In the example depicted in
Figure 5,
the first housing member 90 and the second housing member 92 are provided with
the
threaded holes 124 formed in both the first end portion 108 and the second end
portion I 10
thereof. As depicted in Figures 4-6, the first terminal 116 and the second
terminal 118 are
also provided with threaded fasteners 126 to connect each of the signal paths
75a and 75b to
the first housing member 90 and the second housing member 92.
The first housing member 90 and the second housing member 92 are connected
together by way of any suitable mechanical or chemical type assembly. As
depicted in
17
CA 02528817 2005-12-01
Figure 6, the viscometer-densimeter 60 is provided with a plurality of
threaded fasteners 130
(Figure 6) for securing the first housing member 90 to the second housing
member 92. It
should be noted that the threaded fasteners 130 are typically constructed of
conductive
materials, such as steel or aluminum. To prevent the threaded fasteners 130
from forming
electrical paths between the first housing member 90 and the second housing
member 92, the
viscometer-densimeter 60 is also provided with a plurality of electrically
insulated feed-
throughs 132 to electrically isolate each of the threaded fasteners 130 from
one of the
corresponding first housing member 90 and the second housing member 92.
The sensor unit 62 of the viscometer-densimeter 60 can be anchored within the
evaluation flow line 46 by any suitable assembly. It should be understood that
the sensor unit
62 should be anchored to prevent longitudinal movement within the evaluation
flow line 46
and rotational movement within the evaluation flow line 46. The signal paths
75a and 75b
are preferably provided with sufficient rigidity to prevent longitudinal
and/or rotational
movement of the sensor unit 62 within the evaluation flow line 46. Further
anchoring means
can also be used to prevent movement of the sensor unit 62 within the
evaluation flow line
46. For example, the evaluation flow line 46 can be necked-down downstream of
the sensor
unit 62 so as to prevent longitudinal movement of the sensor unit 62 within
the evaluation
flow line 46.
As will be understood by one skilled in the art, the first housing member 90
and the
second housing member 92, when secured together by way of the threaded
fasteners 130,
cooperate to form the connectors 72. The wire 74 is connected and tensioned as
follows.
The wire 74 is connected at one end. The other end is fed through the second
connector 72
but is not tightened. A mass (not shown) is attached from the end protruding
from the loose
connector 72. The magnitude of the mass, which hangs from the wire 74 within
the Earth's
gravitational field, determines the tension for a wire diameter and therefore
the resonance
frequency; a resonance frequency of about 1 kHz can be obtained with a mass of
500 g
18
CA 02528817 2005-12-01
suspended on a wire of diameter 0.1 mm. The diameter of the wire 74 can be
varied to
change the viscosity range to be measured. After about 24 h, the wire 74 is
clamped at the
second end and the mass removed. This prc)cedure reduces the twist within the
wire 74. The
wire 74 is then heated and cooled so as to produce a wire with a resonance
frequency that is
reasonably stable between each thermal cycle; for the viscometer-densimeter
60, the wire 74
resonance frequency needs to be stable during the time required to determine
the complex
voltage as a function of frequency over the resonance which is on the order of
60 s.
To calculate the viscosity, a sinuioidal current is fed through the wire 74 in
the
presence of a magnetic field. The magnetic field is perpendicular to the wire
74 and in the
presence of the sinusoidal current causes the wire 74 to move. The resulting
induced
electromotive force (motional emf) or complex voltage is added to the driving
voltage. The
motional emf can be detected via the anallrtical circuit 68 with signal
processors that include
lock-in amplifiers, where the driving voltage can be offset or rendered null,
or spectrum
analyzers. When the frequency of the current is close to or at that of the
fundamental
resonance frequency the wire 74 resonat,,-s. The complex voltage is usually
measured at
frequencies over the resonance and the observations combined with the working
equations,
wire density and radius, to determine the viscosity for a fluid of known
density. The
magnitude of the current depends on th,,- viscosity of the fluid and is varied
so that an
acceptable signal-to-noise ratio is obtained with the detection circuitry;
values less than 35
mA are typically used and the resulting complex motional emf of a few
microvolts. In
addition to the magnitude of the current, the diameter of the wire 74 also
determines the
upper operating viscosity; increasing the wire diameter increases the upper
operating
viscosity. There are other ways of exciting and detecting wire motion but none
so convenient
as a lock-in amplifier.
To calculate the viscosity and density of the fluid from the feedback received
from the
wire 74, the analytical circuit 68 operates as follows. The wire 74 is placed
in a magnetic
19
CA 02528817 2005-12-01
field and driven in steady-state transverse oscillations by passing an
alternating current
through it. The resulting voltage V developed across the wire is composed of
two
components:
V =V, +Vz. and (1)
The first term, V1, arises simply from the e;lectrical impedance of the
stationary wire while
the second, V2, arises from the motion of the wire in the presence of the
magnetic field. V 1 is
represented by
V, = a+r'(b+cf), (2)
In equation (2), f is the frequency at which the wire 74 is driven in the
presence of a
magnetic field, while a, b and c are adjustable parameters that are determined
by regression
with experimental results. The parameters a, b and c account for the
electrical impedance of
the wire and also absorb the offset used in the lock-in amplifier to ensure
the voltage signal is
detected at the most sensitive range possible. The second component of V2 is
given in the
working equation of the instrument by
_ Afi
V Z f 2 -(1+~3) f2 +(/.3'+200) f2i (3)
In equation (3), A is an amplitude, fo the resonance frequency of the wire in
vacuum,
Do the internal damping of the wire, P the added mass arising from the fluid
displaced by the
wire, andfl'the damping due to the fluid viscosity.
The fluid mechanics of a vibrating wire that revealed the added mass of the
fluid, (3,
and viscous drag, Q', can be represented by
CA 02528817 2005-12-01
,8= k ~- , and (4)
Ps
,6'=k' P (5)
Ps
where k and k' are given by
k = -1 + 23(A), and (6)
k' = 2~q(A) . (7)
In equations (6) and (7), A is a complex quantity given by
2K, (Ji)
A = i 1 + (8)
S2iKo ( S2i )
where
S2 = w'oRZ (9)
77
In equation (8), Ko and K, are modified Bessel functions and Q is related to
the
Reynolds number that characterizes the flow around the cylindrical wire or
radius R. In
equation (9), the fluid viscosity and density are given by q and p,
respectively. Thus, the
viscosity and density of a fluid can be determined by adjusting the values so
that in-phase and
quadrature voltages predicted from equations (1) through (9) are consistent
with
21
CA 02528817 2005-12-01
experimentally determined values over a function of frequency. The frequency
range over
which data is collected is typically about f+ 5g where g is the half-width of
the resonance
curve and f is the fundamental transverse resonance frequency. In an
electrically perfect
apparatus where the signal-to-noise ratio is large and electrical cross-talk,
which increases
with increasing frequency, zero the band-width selection is not critical.
However, this is
critical when the Q {=f7(2g)} tends to unity that occurs when the band-width
increases,
which it does with increasing viscosity, and, unless the drive current
increased, a
corresponding decrease in signal-to-noise ratio; the importance of determining
the band-
width over which measurements are performed will become apparent below.
Equations (4) through (9) are obtained by assuming the following: (1) the
radius of
the wire 74 is small in comparison with the length of the wire 74, (2) the
compressibility of
the fluid is negligible, (3) the radius of the housing 76 containing the fluid
is large in
comparison to the wire radius so that the boundary effects are negligible, and
(4) the
amplitude of oscillation is small. In the vibrating wire viscometers reported
in the literature,
the resonant frequency is sensitive to both the tension in the wire and the
density of the fluid
that surrounds it; this sensitivity to density is often increased by clamping
the wire at the top
and mounting a mass on the lower end thus invoking Archimedes principle.
However, if the
density is determined from an alternative source, for example, an equation of
state, only the
resonance line width need be stable.
In general, the vibrating wire viscometer, such as the viscometer-densimeter
60, is an
absolute device that, in theory, requires no calibration constants to be
determined. However,
in practice some physical properties of the wire 74 such as density and radius
cannot be
determined to sufficient accuracy by independent methods; hence, those
properties are
usually determined by calibration. To do this, measurements are made in both
vacuum and a
fluid for which the viscosity and density are known. The former yields do. The
wire radius,
R, is the only other unknown variable required to perform viscosity
measurements. The wire
22
CA 02528817 2005-12-01
radius can be determined in a single measurement given the viscosity and
density of the
calibration fluid.
1. Modification of working equations
The complex voltages V developed across the wire 74 consists of Vl arising
from the
electrical impedance of the wire 74 and V2 arising from the motion of the wire
74 in the
presence of the magnetic field (Equation 1). Other than the contribution from
electrical
impedance, V, also accounts for background noise such as electrical cross-talk
or other forms
of coupling. These interference gives rise to a relatively smooth background
over the
frequency interval near the resonant frequency of the vibrating wire 74. In
order to
adequately replicate the measured complex voltages as a function of frequency,
an additional
frequency-dependent parameter is included in Equation (2), i.e.
V, =a+bf+i(c+df). (10)
Without taking the additional frequency-dependent term in Equation (10) into
account, the measured complex voltages are often not fitted well with the
working equations
and consequently, significant errors in fluid density and viscosity are
incurred. This is
particularly true for high viscosity fluids.
2. Determination offluid density and viscosity from vibrating wire
Determination of fluid density and viscosity requires data fitting with the
working
equations of the vibrating wire 74. The method of least squares fitting is
based on the idea
that the optimum characterization of a set of data is one that minimizes the
sum of the squares
of the deviation of the data from the fitting model (or working equations).
The sum of
squares of the deviation is closely related to the goodness-of-fit statistic
called chi-square (or
x2 )
23
CA 02528817 2005-12-01
N
I D(.f;)-v(fr)Z
xZ = "-' , (11)
V
where f is the frequency index, D(fiand v(f) are the recorded complex voltages
and the
working equations, respectively, and v is the number of degrees of freedom for
fitting N data
points. The least squares criterion is formulated as finding the unknown
parameters,
including the fluid density and viscosity, to minimize the chi-square measure
defined in (11),
i.e.
min 2 (12)
P,71,fo,n,Q,d=c,d
where ",d', "rf", ` fo", "A", "a", "b", "c" and "d" are the unknown
parameters. The
Levenberg-Marquardt algorithm [14] provides a nonlinear regression procedure
to solve this
minimization problem.
Among all the unknown parameters, the oscillating amplitude (i.e. A) and the
constants related to electrical impedance of the stationary wire and other
background
interference (i.e. a, b, c and d) are well determined by the minimization
procedure. However,
a fundamental uncertainty among the density, viscosity and fo prevents the
fitting itself from
sorting out the correct density and viscosity values. In order to exterminate
this fundamental
uncertainty, additional relationships among the density, viscosity and fo a
are used as
constraints in the fitting procedure. Mathematically, a relationship among
these variables
can be written in a general functional form
G(P,71,.f0) = 0. (13)
Alternatively, the relationship may also include additional measurements such
as the half-
width of the resonance (g) and the resonant frequency (f ) that can be derived
from the data
H(P,77,ffo, g,= 0. (14)
24
CA 02528817 2005-12-01
Equations (13)-(14) can be established experimentally through calibration
procedures
or empirically based on field data. Our preferred embodiment here is a special
case of
Equations (13)-(14); specifically, a hyper plane defined by a fixedfo. As
discussed in
Retsina et al.(Retsina, T.; Richardson, S. M.; Wakeham, W. A., Applied
Scientific Research,
1987, 43, 325-346; and Retsina, T.; Richardson, S. M.; Wakeham, W. A., 1986,
43, 127-158)
fo can be designated as the resonant frequency of the wire 74 in vacuum that
is directly
related to the tension exerted on the wire 74. 1f fo is known or given, one
can limit the
minimum search on the hyper plane defined by the fixedfo.
Figure 7a shows a flow chart 134 for calculating the viscosity and density
simultaneously as discussed above. Initially, as indicated by block 134a, b
and c, the
constants for wire diameter, wire density, and internal damping factor;
initial estimates for
fluid density, viscosity, and resonant frequencyfo; as well as constraints G
(density, viscosity,
and resonant frequencyfo); are input into a computation block 134d. An initial
wire response
is then computed as represented by the block 134d. The initial wire response
can be
calculated in in-phase and quadrature voltages.
Input data, such as in-phase and quadrature voltages as a function of
frequency are
then received as indicated by a block 134e and the chi-squares are then
computed based on
the difference between the data and computed response as indicated by a block
134f . An
update of the estimates of fluid density, viscosity, and resonant frequency,
lambda, a, b, c and
d are then received. Any non-linear regression analysis can be used to provide
the updates as
indicated by a block 134g. The analytical circuit 68 then applies a convergent
test (as
indicated by a block 134h) based on the chi-squares and the update of the
estimates. If the
convergent test indicates convergence within a predetermined or acceptable
amount, the
process branches to a step 134i where the fluid density and viscosity are
output. However, if
the convergent test indicates convergence outside the predetermined amount,
the process
branches back to the step 134d where the wire response is re-calculated based
on the updated
CA 02528817 2005-12-01
fluid density, viscosity and resonant frequency and the steps 134d, 134e,
134f, 134g and 134h
are repeated until the convergence test indicates convergence within the
predetermined
amount.
Figure 7b shows a flow chart 136 for calculating the viscosity and density
simultaneously, in a manner exactly as described above with respect to Figure
7a, with the
following exceptions. It should be noted that steps in Figure 7b which are
identical to those
in Figure 7a have been labeled with identical reference numerals for purposes
of clarity.
In the process for calculating the viscosity and density represented in Figure
7b, the
sensor unit 62 is tested to determine the resonant frequency fo . To calibrate
the sensor unit
62, the sensor unit 62 is placed in an environmental chamber with a known
fluid, and then the
temperature and pressure are varied so as to provide calibration data. The
calibration data is
then input into the analytical circuit 68 as indicated by a block 136b and
such calibration data
is utilized to compute the resonant frequencyfo as indicated by a block 136c.
Figure 8 is a graph showing the chi-square performance surface intercepted by
the
fixed-fo hyper plane, where there is a global minimum. The graph includes axes
F, D and V.
The F axis represents the frequency offo in Hz. The D axis represents the
density of the fluid
surrounding the wire 74 in kg/m3. The V axis represents the viscosity of the
fluid
surrounding the wire 74 in cp. The meaning of the shading is the value of the
Chi-square -
the dark colors mean a lower chi-square value. The location of a minimum 137
provides the
density and viscosity estimates.
If fo is stable and known within 1 Hz, the fluid density can be determined
within 3-
4 % for a wide range of fluids. The error is smaller (1-2 %) for high
viscosity fluids. If
known within 0.5 Hz, the density error reduces to about 1-2 % for a wide
range of fluids.
The error in viscosity is generally smaller than the density error (about 3 %)
iffo is within 1
Hz. Similarly, the error in viscosity is smaller for high viscosity fluids. To
simultaneously
estimate the fluid density and viscosity, the preferred embodiment requires a
sensor unit
26
CA 02528817 2005-12-01
forming a frequency oscillator for providing a stable and predictablefo in a
wide variety of
different temperatures and pressures. Typical temperature and pressure ranges
in a down
hole environment range from 50 to 200 degrees C and 2.07 to 172.4 MPa (300 to
25000 psi).
Shown in Figure 9 is another version of a sensor unit 150 for use with the
viscometer-
densimeter 60. As will be discussed in more detail below, the sensor unit 150
is similar in
construction and function as the sensor unit 62 described above, with the
exception that the
sensor unit 150 is provided with a pair of conductive connectors 152 separated
by an
insulating flow tube 154 surrounding a wire 156, rather than having the
conductive first
housing member 90 and second housing member 92 separated by a parallel
extending
insulating layer 96. The sensor unit 150 will be described in more detail
below.
The sensor unit 150 forms a frequency oscillator for providing a stable and
predictable fo so that at least two different parameters, such as density and
viscosity of the
fluid in which the sensor unit 150 is immersed can be calculated
simultaneously from the data
generated by the sensor unit 150.
The connectors 152 are designated in Figure 9 by way of the reference numerals
152a
152b for purposes of clarity. The connectors 152 are identical in construction
and function.
Thus, only the connector 152a will be described hereinafter. The connector
152a is provided
with a clamp member 158, a clamp plate 160, and at least one fastener 162 for
connecting the
clamp plate 160 to the clamp member 158. The clamp member 158 is connected to
the flow
tube 154 via any suitable mating assembly. For example, as shown in Figure 9,
the clamp
member 158 is provided with an end support 166 that mates with a predetermined
portion of
the flow tube 154 such that the end support 166 is supported by the flow tube
154. In the
version depicted in Figure 9, the flow tube 154 is provided with a necked down
portion 168,
and the end support 166 defines a collar positioned over the necked down
portion 168. The
clamp member 158 is also provided with a flange 170 connected to and extending
from the
27
CA 02528817 2005-12-01
end support 166. To center the wire 156 on the flange 170, at least one
registration pin 174 is
provided on the flange 170. Desirably, the clamp member 158 is provided with
at least two
spaced-apart registration pins 174 such that the wire 156 can be threaded
between the
registration pins 174 as shown in Figure 9.
The fasteners 162 connect the clamp plate 160 to the clamp member 158 so as to
clamp the wire 156 thereto. The fasteners 162 can be any type of device
capable of
connecting the clamp member 158 to the clamp plate 160. For example, the
fastener 162 can
be a screw.
The flow tube 154 is preferably constructed of a material which has a similar
coefficient of thermal expansion as the wire 156. When the wire 156 is
constructed of
tungsten, the flow tube 154 can be constructed of a ceramic, such as Shapal-M.
At least one opening 180 is formed in the clamp member 158 to permit fluid to
enter
or exit the flow tube 154 through the opening 180. As shown in Figure 9, the
clamp member
158 can be provided with at least two openings 180 with each opening 180
having a
semicircular shape. However, it should be understood that the shape of the
openings 180 can
vary depending on the desires of the designer. More specifically, it should be
understood that
the openings 180 can have any asymmetrical, symmetrical or fanciful shape.
The wire 156 is constructed in a similar manner to the wire 74 discussed
above. The
wire 156 is supported and tensioned within the flow tube 154 in a similar
manner as the wire
74 is supported and tensioned within the housing 76. The signal paths 75a and
75b from the
signal processor 66 and the analytical circuit 68 are connected to the
respective connectors
152 in any suitable manner, such as screws, bolts, terminals or the like.
As discussed above, iffo, the resonance in vacuum of equation (1), of the
sensor unit
150 is stable, then it is possible to determine both density and viscosity
from the measured
complex voltages as a function of frequency over the resonance. Because the
sensor unit 150
includes two metallic connectors 152 separated by the flow tube 154 formed
from an
28
CA 02528817 2005-12-01
electrically isolating material; these materials have different elastic and,
in some cases, also
thermal properties. The connectors 152 and the flow tube 154 are preferably
held together
solely by the tension of the wire 156.
The sensor unit 150 preferably has an fo unaffected by the fluid properties
and
pressure. The latter may have a small but yet calculable contribution from the
wire material
compressibility. In addition, the response of the wire 156 to temperature
variations, that
include differential thermal expansion arising from the use of dissimilar
materials in the
construction of the resonator, should either be measurable or calculable. The
wire 156 is
tensioned and set into transverse motion by passing an electrical current
through it in the
presence of a perpendicular magnetic field. These factors imply that the
sensor unit 150
could be improved by eliminating rotational motion of the wire 156 that might
arise from the
wire 156 having an elliptical cross-section, and the sensor unit 150 must also
electrically
isolate each end of the wire 156 to permit current to flow through it.
Tungsten, despite the surface roughness, is the preferred material for the
wire 156 for
measurements involving liquid because both Youngs' modulus E(;:Z~ 411 GPa) and
density ps
(zz~ 19,300 kg=m-3) are high relative to other materials. When the wire 156 is
tensioned the
former assists in providing a stable resonance while the latter provides
sensitivity to the fluid
around it, through the ratio p/ps in equations (4) and (5). The effect of
surface roughness is
negligible provided the amplitude of vibration is small and Reynolds number
less than 100.
For measuring the density, it is desirable that the wire density tend toward
the density of the
fluid; derived from added mass concepts. Thus, Tungsten can be used but other
materials of
lower density are also acceptable depending upon the expected density of the
fluid to be
measured.
To minimize the effect of differential thermal expansion, this choice of wire
material
dictates the material to use for the connectors 152, flow tube 154 and
tensioning mechanism.
It is desirable that the mechanical properties of the electrically insulating
material forming the
29
CA 02528817 2005-12-01
flow tube 154 be as close as possible to those of the materials used for both
the wire 156 and
the connectors 152. For example, the effect of differential thermal expansion
on the wire
tension, as the temperature departs from ambient, could be reduced by choosing
a material
with linear thermal expansion coefficient equivalent to that of tungsten;
Shapal-M, which is a
high thermal conductivity machinable ceramic with a compressive strength of 1
GPa, has a
linear thermal expansion coefficient a=(1/L)dL/dT = 5.2= 10-6 K-t at T= 298 K
while a(W,
298 K) ;:t~ 4.5=10-6 K-'. Alternative materials for the insulating material
might include either
aluminum nitride or Macor, however, a for these materials is not equivalent to
W.
The criteria described in the preceding paragraph were used to formulate
another
version of a sensor unit 200 for a vibrating wire viscometer-densimeter 60
shown in Figures
11 and 12 to reduce the variation in fo arising from temperature, pressure,
and fluid
properties. The sensor unit 200 is similar in construction and function as the
sensor unit 150,
with the exception that the temperature and pressure effects are reduced by
constructing the
sensor unit 200 mostly from a same material, such as tungsten, which has the
same thermal
expansion and elastic properties, while also minimizing the rotation of a wire
156 to reduce
the effect on fo arising from variations in fluid properties. The sensor unit
200, shown in
Figure 11, consists of two connectors 204 and 206, both formed from tungsten
and a flow
tube 208 positioned between the connectors 204 and 206 within which the wire
202 is held.
The wire 202 is rigidly connected to each connector 204 and 206. For example,
in the
example shown in Figures 11 and 12, the wire 202 is electron beam welded (EBW)
to each
connector 204 and 206.
The connector 204 includes a boss 212 and an end-piece 214. The boss 212 is
connected to the wire 202 and is designed to prevent rotation of the wire 202.
For example,
the boss 212 can be provided with a non-circular cross-section, e.g., square,
to prevent
rotation of the wire 202. The boss 212 is positioned within a cavity formed in
the end-piece
214. The boss 212 is shaped to facilitate alignment with the connector 206.
The boss 212
CA 02528817 2005-12-01
can be formed from any shape suitable for facilitating alignment with the
connector 206. For
example, the boss 212 can include a tapered or conical end to facilitate
alignment with the
connector 206. The wire 202 can be attached to the boss 212 via any suitable
manner that
rigidly fixes the wire 202 to the boss 212. For example, the wire 202 can be
positioned
within a slot (not shown) formed in the boss 212 and electron beam welded as
described
above such that the boss 212 forms a clamp about the wire 202.
The connector 206 is provided with an end-mount 216, a boss 218, an insulator
220
and an adjustment assembly 222 for adjusting the relative positions of the
boss 218 and the
end-mount 216. The boss 218 is connected to the wire 202 in the same manner as
the boss
212 is connected to the wire 202. The boss 218 is designed to prevent rotation
of the wire.
For example, the boss 218 can be provided with a non-circular cross-section,
e.g., square, to
prevent rotation of the wire 202. The boss 218 is positioned within a cavity
224 formed in
the end-piece 216.
The insulator 220 provides electrical isolation between the end-piece 216 and
the boss
218. In the embodiment shown in Figures 11 and 12, the insulator 220 is formed
as a sleeve
lining the cavity 224 within the end-piece 216 and extending across a face 226
of the end-
piece 216. The insulator 220 can be formed of any insulating material capable
of
withstanding a down hole environment. For example, the insulator 220 can be
constructed of
a ceramic material, such as Shapal-M.
The adjustment assembly 222 can be any device capable of adjusting the
relative
positions between the boss 212 and the end-piece 216 to permit adjustment of
the tension in
the wire 202. For example, the adjustment assembly 222 can include a wire
tensioning nut
230 that is threaded to the boss 212. Of course, there are many other
arrangements that could
be used to clamp the wire 202 to a housing to permit tensioning of the wire
202. For
example, between two clamps or connectors as shown, or the use of a spring.
As discussed above, it is desirable for the tensioned vibrating wire 74, 156
or 202 to
31
CA 02528817 2005-12-01
have a stable resonance frequency with respect to temperature, pressure and
fluid. A stable
resonance frequency essentially reduces to a requirement of constant wire
tension. Although
it is plausible to construct a stable oscillator solely from mechanical
considerations, another
solution is afforded by the concept of relative measurements. Shown in Figure
13 is a
fragmental view of another version of a down hole 10a which is similar in
construction and
function to the down hole 10, discussed above, except that the down hole tool
l0a has two or
more viscometer-densimeters 60 with one of the viscometer-densimeters 60
(designated as
60a) positioned within a fluid of unknown viscosity and density and another
one of the
viscometer-densimeters 60 (designated as 60b) positioned within a fluid of
known viscosity
and density. Each of the viscometer-densimeters 60a and 60b are provided with
magnets 64a,
64b. In this approach, two similar sensor units 250a and 250b are used with
one immersed in
the fluid of unknown properties, e.g., density and viscosity, and the other in
the fluid of
known properties. The sensor units 250a and 250b can be constructed in a
manner described
above with respect to the sensor units 62, 150 or 200 described above.
The sensor unit 250a is positioned within an evaluation flow line 252, which
can be
the evaluation flow line 46, the cleanup flow line 46a or the sample chamber
50 discussed
above. In the down hole tool 10a, an elbow or joint 254 is provided that is in
fluid
communication with the flow line 252. The joint 254 defines a comparison
chamber 255 in
which the known fluid and the sensor unit 250b are positioned. The down hole
tool 10a is
provided with a pressure equalization assembly 256 for equalizing the pressure
within the
evaluation flow line 252 . In general, the pressure equalization assembly 256
can be any
device capable of equalizing the pressure between the evaluation flow line 252
and the
comparison chamber 255. For example, as shown in Figure 13, the pressure
equalization
assembly 256 can include a reciprocating piston 258 which moves relative to
the comparison
chamber 255 to equalize the pressure.
The sensor units 250a and 250b are connected to one or more signal processor
260
32
CA 02528817 2005-12-01
and analytical circuit 262 for providing the drive voltage and determining one
or more fluid
parameters, such as viscosity and density, as discussed above. The signal
processor 260 and
the analytical circuit 262 are similar in construction and function to the
signal processor 66
and the analytical circuit 68 discussed above.
The ratio of the resonances between the sensor units 250a and 250b are
determined as
illustrated, for example, in Figures 14a and 14b. Figure 14a shows a process
170 for
calculating the density and viscosity of the fluid utilizing the dual
viscometer-densimeters
60a and 60b illustrated in Figure 13. The process 170 has similar steps to
those utilized in
Figure 7a discussed above. For purposes of clarity, the similar steps are
labeled with the
same reference numerals 134a, 134b, 134d, 134e, 134f, 134g, 134h and 134i and
will not be
described in detail again.
In general, the density and the viscosity of the fluid which will be in the
comparison
chamber 255 is determined by known methods, such as using tables from the
United States
National Institute of Standards and Technology (NIST) as indicated by steps
172 and 174.
The analytical circuit 262 receives signals from the sensor unit 250b as
indicated by a step
176, and then calculates the resonant frequency based on the known density and
viscosity of
the fluid within the comparison chamber 255 as indicated by a step 178. The
analytical
circuit 262 then computes the viscosity and density in the manner described
above with
respect to Figure 7A.
Shown in Figure 14B is another process 180 for computing the fluid density and
viscosity of the unknown fluid within the flow line 252. In the process 180,
initial estimates
of the fluid density, viscosity and lambda a, b, c, and d are input into the
analytical circuit 262
as indicated by blocks 182 and 183. Constants, such as wire diameter, wire
density and
internal damping factor are input into the analytical circuit 262 as indicated
by a block 184.
Other inputs, such as the temperature and pressure that the sensor unit 250a
in the flow line
252 is being exposed to are input into the analytical circuit 262 as indicated
by a block 186.
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CA 02528817 2005-12-01
The input data, such as in-phase and quadrature data, are then read from the
sensor units 250a
and 250b as represented by blocks 188 and 190 and a joint inversion of the
data from sensor
250a and 250b is computed as indicated by the block 183. The analytical
circuit 262 then
outputs the fluid density and the viscosity of the fluid surrounding the
sensor unit 250a as
indicated by a block 192.
Although the two foregoing methods for calculating viscosity and density have
been
described above, it should be understood that any manner could be utilized,
such as a ratio
measurement of the outputs generated by the two sensor units 250a and 250b.
Provided the wires within the sensor units 250a and 250b are of similar
construction
(preferably identical construction) and exposed to the same temperature and
pressure any
instabilities arising from these variables is eliminated and data is obtained
indicative of a
stable oscillator. If both concepts are combined, that is a comparison or
ratio measurement
and a stable geometry as recited above with respect to the sensor units 150
and 200, then it is
plausible that the resonator will be stable and be able to provide both
density and viscosity.
It will be understood from the foregoing description that various
modifications and
changes may be made in the preferred and alternative embodiments of the
present invention
without departing from its true spirit. The devices included herein may be
manually and/or
automatically activated to perform the desired operation. The activation may
be performed as
desired and/or based on data generated, conditions detected and/or analysis of
results from
down hole operations.
This description is intended for purposes of illustration only and should not
be
construed in a limiting sense. The scope of this invention should be
determined only by the
language of the claims that follow. The term "comprising" within the claims is
intended to
mean "including at least" such that the recited listing of elements in a claim
are an open
group. "A," "an" and other singular terms are intended to include the plural
forms thereof
unless specifically excluded.
34
